1. Field of the Invention
The present invention relates generally to printed wiring boards, and more particularly it relates to a double-sided printed wiring board on which semiconductor-based integrated circuits (referred hereinafter to as "ICs") may be mounted, and a process of manufacturing such double-sided printed wiring boards.
2. Description of the Prior Art
Typically, a conventional printed wiring board carries ICs as well as other discrete electronic components and circuit elements, which are interconnected to provide the particular electronic circuit functions. In the prior art, those ICs, discrete electronic components and circuit elements are usually bonded to the printed wiring board via holes formed in the printed wiring board through which lead wires may be inserted and soldered to the board. However, there have been advances in surface mounting technology widely employed in the printed wiring board manufacturing field. This technology permits an IC to be mounted together with its associated elements on the printed wiring board without forming any through-holes in the board. Thus, ICs and other on-chip elements may be mounted on a chip land directly without using the through-holes.
To provide for the interconnections between the on-chip elements on the chip land on one side and any circuit on the opposite side of the printed wiring board, the appropriate through-via-holes must be provided remotely from the chip land, and any wiring pattern required for interconnecting the elements via the through-via-holes must be formed on the surface of the base plate.
Thus, according to the prior art, the chip land and through-via-holes must be provided at different locations on the printed wiring board. As the size of each of the IC and other elements is reduced, a corresponding reduction in the size of the chip land is required so that they can be mounted on the chip land. The wiring pattern that includes leads drawn out from the chip land and distributed across different locations must be accordingly fine, but technically, this is practically difficult to achieve. It is also difficult to secure the space required for wiring the leads. In particular, for a double-sided high-density wiring pattern implementation, this space limitation poses a problem.
The through-via-holes formed in the printed wiring board are exposed on each of the opposite sides of the board. When leads are inserted through the corresponding through-via-holes, and the associated circuit components are fixed by the board in solder, surplus solder may flow through the through-via-holes, thereby reaching the components on the chip land.